1. Field of the Invention
The present invention relates to semiconductor devices, and particularly to a programmable logic device and a semiconductor device that can change a hardware configuration.
2. Description of the Related Art
A programmable logic device (PLD) includes a plurality of programmable logic elements and a plurality of programmable switch elements. A circuit structure of a PLD can be varied to change its function in such a manner that a function of each programmable logic element or a connection between programmable logic elements through programmable switch elements is changed by programming performed by a user after fabrication.
A function of each programmable logic element and a connection between programmable logic elements through programmable switch elements are determined by configuration data. Configuration data is stored in a memory circuit included in a programmable logic element or a memory circuit included in a programmable switch element. A memory circuit for storing configuration data is hereinafter referred to as configuration memory.
Patent Document 1 suggests a programmable logic device using, as configuration memory, memory that is composed of a combination of a transistor in which a semiconductor layer including a channel formation region contains an oxide semiconductor (also referred to as OS transistor) and a transistor in which a semiconductor layer including a channel formation region contains silicon (also referred to as Si transistor). The use of the memory including the OS transistor and the Si transistor as configuration memory makes it possible to configure a nonvolatile programmable switch with a small number of transistors.
Moreover, a semiconductor device in which data is retained by a combination of a Si transistor and an OS transistor has attracted attention (see Patent Document 2).